Calculation of alkane to water solvation free energies using continuum solvent models

The FDPB/gamma method and the PARSE parameter set have been recently shown to provide a computationally efficient and accurate means of calculating hydration free energies.' In this paper this approach is extended to the treatment of the partitioning of various solute molecules between the gas phase, water, and alkane solvents. The FDPB/gamma method treats the solute molecule as a polarizable cavity embedded in a dielectric continuum. The solute charge distribution is described in terms of point charges located at atomic nuclei. Electrostatic free energies are obtained from numerical (finite difference) solutions to the Poisson (or Poisson-Boltzmann) equation, while nonpolar contributions are treated with a surface area-dependent term proportional to a surface tension coefficient, gamma. To apply the FDPB/gamma method to nonaqueous phases, it is necessary to derive a continuum representation of solute-solvent interactions appropriate for such systems. It is argued in this work that solute cavities in nonpolar solvents are significantly larger than in aqueous media. The physical basis for the existence of an expanded cavity in nonpolar solvents is discussed. When an expanded cavity, described in terms of increased values for atomic radii, is incorporated into the FDPB/gamma formalism, good agreement between calculated and experimental solvation free energies is obtained. A new PARSE parameter set is developed for the transfer of organic molecules' between alkanes and water which yields an average absolute error in solvation free energies of 0.2 kcal/mol for the 18 small molecules for which the parameters were optimized.